I imagine either editor Julius Schwartz or Len Wein (the guy who wrote the first installment) came up with the basic "12 Labors" concept. Once that was established, all you'd have to do is decide who the JLA guest-star would be for each issue, and let the writer run with it from there. It's possible that Schwartz deliberately set things up this way so he could "audition" various writers to see who he wanted to take over permanently.